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Tuning the Exospace Weather Radio for Stellar Coronal Mass Ejections

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Tuning the Exospace Weather Radio for Stellar Coronal Mass Ejections The Astrophysical Journal, 895:47 (10pp), 2020 May 20 https://doi.org/10.3847/1538-4357/ab88a3 © 2020. The American Astronomical Society. All rights reserved. Tuning the Exospace Weather Radio for Stellar Coronal Mass Ejections Julián D. Alvarado-Gómez1,2,8 , Jeremy J. Drake2 , Federico Fraschetti2,3 , Cecilia Garraffo2,4 , Ofer Cohen5 , Christian Vocks1 , Katja Poppenhäger1 ,Sofia P. Moschou2 , Rakesh K. Yadav6 , and Ward B. Manchester IV7 1 Leibniz Institute for Astrophysics Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany; [email protected] 2 Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA 3 Dept. of Planetary Sciences-Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, 85721, USA 4 Institute for Applied Computational Science, Harvard University, Cambridge, MA 02138, USA 5 University of Massachusetts at Lowell, Department of Physics & Applied Physics, 600 Suffolk Street, Lowell, MA 01854, USA 6 Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA 7 Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109, USA Received 2020 March 5; revised 2020 April 9; accepted 2020 April 10; published 2020 May 22 Abstract Coronal mass ejections (CMEs) on stars other than the Sun have proven very difficult to detect. One promising pathway lies in the detection of typeII radio bursts. Their appearance and distinctive properties are associated with the development of an outward propagating CME-driven shock. However, dedicated radio searches have not been able to identify these transient features in other stars. Large Alfvén speeds and the magnetic suppression of CMEs in active stars have been proposed to render stellar eruptions “radio-quiet.” Employing 3D magnetohydrodynamic simulations, we study the distribution of the coronal Alfvén speed, focusing on two cases representative of a young Sun-like star and a mid-activity M-dwarf (Proxima Centauri). These results are compared with a standard solar simulation and used to characterize the shock-prone regions in the stellar corona and wind. Furthermore, using a flux-rope eruption model, we drive realistic CME events within our M-dwarf simulation. We consider eruptions with different energies to probe the regimes of weak and partial CME magnetic confinement. While these CMEs are able to generate shocks in the corona, those are pushed much farther out compared to their solar counterparts. This drastically reduces the resulting type II radio burst frequencies down to the ionospheric cutoff, which impedes their detection with ground-based instrumentation. Unified Astronomy Thesaurus concepts: Magnetohydrodynamical simulations (1966); Solar flares (1496); Stellar flares (1603); Solar coronal mass ejections (310); Solar coronal mass ejection shocks (1997); Stellar winds (1636); Solar wind (1534); Solar radio emission (1522); Stellar magnetic fields (1610); Solar magnetic fields (1503); Stellar coronae (305); Radio bursts (1339) 1. Introduction populations (e.g., Davenport 2016; Guarcello et al. 2019; Ilin et al. 2019). This wealth of information is increasingly driving Flares and coronal mass ejections (CMEs) are more energetic the study of their effects on exoplanet atmospheres (e.g., than any other class of solar phenomena. These events involve ) ( ) 33 Segura et al. 2010; Mullan & Bais 2018; Tilley et al. 2019 . the rapid seconds to hours release of up to 10 erg of On the other hand, the observational evidence for stellar CMEs magnetic energy in the form of particle acceleration, heating, ( is very thin, with a single direct detection of an extreme event radiation, and bulk plasma motion Webb & Howard 2012; recently reported by Argiroffi et al. (2019) using Chandra. ) ( Benz 2017 . Displaying much larger energies by several Unfortunately, current X-ray instrumentation renders the metho- ) orders of magnitude , their stellar counterparts are expected to dology of this detection—resolving (temporally and spectro- play a fundamental role in shaping the evolution of activity and scopically) apost-flare blueshift signature in cool coronal lines— rotation (Drake et al. 2013; Cranmer 2017; Odert et al. 2017), sensitive only to the most energetic eruptions. As described by as well as the environmental conditions around low-mass stars Moschou et al. (2019), other diagnostics, such as asymmetries in (see e.g., Micela 2018). Energetic photon and particle radiation Balmer lines or continued X-ray absorption during flares, have associated with flares and CMEs are also the dominant factors provided only a handful of good CME candidates so far (see also driving the evaporation, erosion, and chemistry of protoplane- Moschou et al. 2017;Vidaetal.2019). tary disks (e.g., Glassgold et al. 1997; Turner & Drake 2009; Analogous to the Sun, an alternative way of recognizing Fraschetti et al. 2018) and planetary atmospheres (e.g., CMEs in distant stars lies in the detection of the so-called type Lammer 2013; Cerenkov et al. 2017; Tilley et al. 2019). This II radio bursts (Wild & McCready 1950; Kundu 1965; Osten & is critical for exoplanets in the close-in habitable zones around Wolk 2017). These transients correspond to two distinct bright M-dwarfs, which are the focus of recent efforts to locate nearby lanes in radio spectra in the kHz–MHz range, separated by a habitable planets (Nutzman & Charbonneau 2008; Tuomi et al. factor of ∼2 in frequency, characterized by a gradual drift from 2019), but are known for their long sustained periods of high high to low frequencies. These features are attributed to flare activity (see Osten 2016; Davenport et al. 2019). emission at the fundamental and first harmonic of the plasma 9 Stellar flares are now routinely detected across all wave- frequency, resulting from non-thermal electrons accelerated by lengths from radio to X-ray, spectral types from F-type to brown dwarfs, and ages from stellar birth to old disk 9 -12 Expressed in c.g.s. units as npp = ()(24 peme ) n, where e, m are the electron charge and mass, and n denotes the number density of the ambient 8 Karl Schwarzschild Fellow. region. 1 The Astrophysical Journal, 895:47 (10pp), 2020 May 20 Alvarado-Gómez et al. a shock generated as the velocity of the CME in the stellar wind Furthermore, we test the hypothesis of radio-quiet CMEs in frame surpasses the local Alfvén speed (i.e., U CME- V SW > M-dwarfs by simulating eruptions in the regimes of weak and SW fi fi VA = B 4pr , with V , B and ρ as the stellar wind speed, partial large-scale magnetic eld con nement, and determine magnetic field strength and plasma density, respectively10). The whether or not they become super-Alfvénic during their frequency drift reflects the decrease in particle density with temporal evolution. distance as the CME shock propagates outward in the corona This paper is organized as follows: Section 2 contains a (see Cairns et al. 2003; Pick et al. 2006 and references therein). description of the employed models and boundary conditions. While solar observations indicate an association rate of just Results from the steady-state Alfvén speed distributions, as ∼1%–4% between CMEs and type II bursts in general (see well as the time-dependent M-dwarf CME simulations, are Gopalswamy et al. 2005; Bilenko 2018), it is close to 100% for presented in Section 3. We discuss our findings and their the most energetic eruptions (Gopalswamy et al. 2009, 2019). implications in Section 4, and provide a brief summary in Furthermore, the high fraction of CMEs associated with strong Section 5. flares in the Sun (∼80%–90% for X-class flares, see Yashiro & Gopalswamy 2009; Compagnino et al. 2017), combined with 2. Models fl ( the enhanced stellar are rates and energies e.g., Kashyap et al. The numerical simulations discussed in this work follow 2002; Caramazza et al. 2007; Shibayama et al. 2013; Loyd closely the methodology described in Alvarado-Gómez et al. ) et al. 2018 , are expected to yield enough type II burst events in (2018, 2019b), where different models included in the Space active stars to secure detections. While several other radio Weather Modeling Framework11 (SWMF, Gombosi et al. transients have been detected in low-mass stars (see e.g., 2018) were used. Villadsen & Hallinan 2019), this particular class of radio event has not been observed so far (e.g., Crosley et al. 2016; 2.1. Steady-state Configurations Villadsen 2017; Crosley & Osten 2018a, 2018b). However, based on solar statistics it is clear that the lack of typeII bursts The corona and stellar wind solutions are based on the 3D ( ) does not imply an absence of CMEs. MHD code BATS-R-US Powell et al. 1999; Tóth et al. 2012 and the data-driven Alfvén Wave Solar Model (AWSoM, van Recent numerical studies have started to provide a common ) framework to interpret these observations and the apparent der Holst et al. 2014 . The latter, extensively validated and fl updated against remote and in situ solar data (e.g., Oran et al. imbalance between are and CME occurrence in stars. Detailed ) MHD models have shown that the stellar large-scale magnetic 2017; Sachdeva et al. 2019; van der Holst et al. 2019 , field can establish a suppression threshold preventing CMEs of considers Alfvén wave turbulent dissipation as the main driver of coronal heating and stellar wind acceleration. Both certain energies from escaping (Drake et al. 2016; Alvarado- contributions are self-consistently calculated and incorporated Gómez et al. 2018). The rationale of this mechanism comes as additional source terms in the energy and momentum from solar observations, where it has been proposed to operate equations, which, combined with the mass conservation and on smaller scales, forming a magnetic cage for the plasma magnetic field induction equations, close the non-ideal MHD ejecta in certain flare-rich CME-poor active regions (e.g., Sun ) set of equations solved numerically. Radiative losses and et al.
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